PrintSpooler: Require empty output for PDF

When saving to PDF, request a truncated output stream in case the user
selects to overwrite an existing file.

Bug: 423815728
Test: Overwrite existing PDF
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:d65493c65a94415208d4be2454181f214fb2ed03)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:bfd77aeddc4d3b960a1393ad549e67152179ab1b)
Merged-In: Id598cd1a9d2456566f3905432eb7b5ffeab9d33e
Change-Id: Id598cd1a9d2456566f3905432eb7b5ffeab9d33e